The domain of applicability of wireless communications is expanding from transmission media for telecommunications to cableless connection means at a range as close as several centimeters. In fact, short-range and/or close-range wireless communication schemes, including Bluetooth and wireless LAN, have already been put to practical use. Such communication schemes are superior in adaptive flexibility. Under these circumstances, it is expected that in the near future wireless communication functions are to be built in all kinds of equipment and devices.
Along with diversification of communication systems, a single-system communication scheme based on a global standard, such as GSM or IMT-2000, is shifting to a multi-system communication scheme in which different systems for different purposes coexist to build up flexible communications. In accordance with the mobile multi-system environment, a software defined radio (a so-called “clever radio”) is being developed. A software defined radio is a radio whose functions (including channel modulation waveforms) are defined by overwriting software in a single radio transmitter/receiver (hardware) so as to be applicable to various wireless schemes. This technology allows adaptive connection to different network systems, and sufficiently responds to application-oriented communications.
One of the obstacles in the way of this technical innovation is shortage of spectrum. One important reason for the shortage of spectrum is the licensing-based governing structure. Under the current spectrum management scheme, radio waves are exclusively allocated to telecommunication carriers to bring them under control. Each of the carriers has to deal with traffic fluctuation using only the licensed spectrum.
To alleviate such circumstances, a concept of “open-spectrum” for sharing the radio spectrum among users, as well as reorganization of the spectrum, has been proposed.
For example, WO97/09838 (Publication 1) discloses a method for sharing unrestricted frequency bands among different wireless communication systems. To be more precise, a first frequency band is allocated to a first wireless communication system, and a second frequency band is allocated to a second wireless communication system within a prescribed range. A non-allocated frequency band in that range is shared by the first and second communication systems without synchronization.
JP 2003-101506A (Publication 2) discloses a wireless communication terminal device that uses different types of frequency bands depending on the communication environment in dual mode wireless communication. For example, the device uses the 2.4 GHz band outside, which band is not subjected to restriction of the Radio Law, while using the 5.2 GHz band indoors. In dual-mode wireless communications using different frequency bands, different modulation schemes are employed generally. However, in this publication, the device employs a common modulation scheme by appropriately adjusting the parameters, thereby realizing dual-mode communication.
On the other hand, in a wireless communication system using an adaptive modulation scheme in a single spectrum band, slot allocation is carried out so as to give a higher priority to a terminal device with a higher transmission ability during the transmission scheduling among terminal devices with different transmission abilities in order to prevent a shortage of slots due to an increase of the number of wireless terminal devices. This arrangement can improve the overall throughput of the system. See, for example, JP 2003-18647A (Publication 3).
The above-described Publication 1 (WO97/09838) aims to increase the efficiency of spectrum use as a whole by using an unused spectrum band shared among different systems, in addition to an exclusively allocated spectrum band.
Publication 2 (JP 2003-101506A) aims to appropriately use a restricted spectrum band and an unrestricted spectrum band (provided for services that do not require license from the government), while preventing an increase of hardware by appropriately selecting parameters according to the spectrum band being used.
Both publications employ a concept of parallel use or sharing of different spectrum bands. However, the spectrum bands used in this publication are very limited and fixed, and there is no technical approach to adaptive allocation of a currently available optimum spectrum band and/or the optimum channel in that spectrum band taking into account the user QoS and fairness among users.
In addition, neither publication refers to optimization of transmission parameters according to the user QoS, while following the characteristic differences (as to propagation loss or the available bandwidth) among different spectrum bands, and optimization of transmission scheduling among multiple users.
Publication 3 is addressed to allocation of resources to different types of terminal devices with different performance levels in a single spectrum band. Accordingly, this technique cannot be applied as it is to the multiband environment where multiple separate spectrum bands are used by a single system or shared by different systems. Publication 3 does not address maximization of the system efficiency taking the requested QoS and characteristic differences among separate spectrum bands into account in the multiband mobile communication environment.
It is expected that in the future the radio frequency spectrum management policy will be revised, and that a registration system for registering those operators who meet the standard is going to be employed, besides the current licensing system that allows exclusive license of a specific spectrum. In addition, unlicensed spectrum bands may be further opened up in the future. From this viewpoint, consideration has to be given to coexistence of different types of telecommunication carriers, including TV networks, broadcasters, and mobile communication services, as well as to reduction of interference and efficient allocation of available spectrum bands and/or channels in each spectrum band.